Are you certain you have done everything that is required of you to descend to the crypt? There's at least four things you need to do before you can.

spoiler:

Align the moons in the planetarium.
Place the blue crystal from the power room under the nexus into the alcove.
Place the metal rod from the critter's house (I that right?) over the panel on the floor.
Dust off the lens outside the entrance chamber.